Protease activated receptor-2 (PAR2) is a unique G-protein coupled receptor (GPCR) in that it has no known endogenous extracellular ligand, but rather is activated by proteases including many serine proteases such as trypsin, tryptase, and cathepsin G. Serine proteases cleave a section of the membrane bound receptor's extracellular N-terminus, exposing a new sequence which acts as a tethered ligand by binding to the receptor and initiating activation.
Distributed widely throughout the body, PAR2 has been implicated as a pro-inflammatory mediator in acute and chronic inflammatory diseases including arthritis, inflammatory bowel disease, pancreatitis, and cardiovascular diseases. PAR2 has also been reported as anti-inflammatory and protective in conditions such as gastric ulcer, colitis, asthma, and liver fibrosis, although this remains controversial. PAR2 activation has been linked to proliferation, metastasis and angiogenesis in many cancers including cancers of the stomach, colon, breast and pancreas. In this context, small-molecule modulators of PAR2 are of potential interest as a new class of anti-inflammatory, pro-inflammatory, anti-proliferative or proliferative agents.
Trypsin is a potent activator of PAR2 in the GI tract where pancreatic trypsin is found, and in colon, airway epithelium, neuronal and vascular endothelial cells, skin, intestine, kidney and pancreas where trypsinogen expression has been demonstrated. Mast cell tryptase is also an important activator of PAR2, being highly expressed in mast cells and strongly associated with many inflammatory, endocrine and other diseases. Hexapeptides SLIGKV-NH2 and SLIGRL-NH2, corresponding to the tethered ligand human and murine sequences respectively, can activate human PAR2 in lieu of serine proteases, albeit at lower potency (μM instead of nM concentrations).
More potent peptide agonists have been created for PAR2. The hexapeptide analogue. 2-furoyl-LIGRL-NH2, has ˜20-fold higher agonist potency than SLIGRL-NH2 and is selective for PAR2 over PAR1. Other heterocyclic replacements for serine result in equipotent PAR2 agonists, while large aromatic groups in place of the C-terminal leucine impart a similar enhancement in PAR2 agonist potency (McGuire, J. J. et al. J Pharmacol Exp Ther 2004, 309, 1124-31; Barry G D et al, Bioorg Med Chem 2007, 27, 5552-7; Hollenberg, M. D., et al, J Pharmacol Exp Ther 2008, 326, 453-62; Boitano. C. et al, J Med Chem 2011, 54, 1308-13; Flynn, A. N., et al, J Biological Chem 2011, 286, 19076-88). Screening of 250,000 drug-like compounds produced two small molecule agonists of PAR2 with similar agonist potency to 2-furoyl-LIGRL-NH2, some selectivity for PAR2 and metabolic stability in vivo (Seitzberg, J. G., et al. J Med Chem 2008, 51, 5490-3).
The first known antagonist of PAR2 had affinity at only millimolar concentrations for the receptor and selectivity is most unlikely (Kelso. E. B., et al. J Pharmacol ExpTher 2006. 316, 1017-24). A second antagonist reported for PAR2 is active at μM concentrations, but completely inactive against endogenous PAR2 activators like trypsin (Kanke, T. et al. Br J Pharmacol 2009, 158, 361-371) or has a dual function as an antagonist and agonist due to either partial agonist actions or possible agonist-directed signalling (Goh, F. G., et al. Br J Pharmacol 2009, 158, 1695-1704).
In one or more aspects, the present invention may advantageously provide a novel class of compounds that can selectively modulate PAR2 when used at low micromolar or sub-micromolar concentrations. Depending upon structural characteristics, and intracellular pathways being examined, these novel compounds may act as either agonists or antagonists and be useful as tools for biological studies or as agents for anti-inflammatory, pro-inflammatory, anti-proliferative or proliferative therapies.